(1) Field of the Invention
The present invention relates to high energy density electrochemical cells containing improved electrolyte compositions. More specifically, the present invention is directed to rechargeable, high energy density electrochemical cells having alkali metal anodes and having electrolyte compositions containing electrolytically active alkali metal salt complexes including haloorganometallic alkali metal salt complexes.
(2) Prior Art
A recently developed rechargeable, high energy density electrochemical cell consists of an alkali metal material as the anode-active material, a transition metal chalcogenide as the cathode active material, and a nonaqueous electrolyte. More specifically, preferred cells consist of lithium anodes, titanium disulfide cathodes and nonaqueous electrolyte compositions consisting of various lithium salts, such as LiClO.sub.4, dissolved in organic solvents, such as propylene carbonate, tetrahydrofuran, dioxolane, and mixtures of dimethoxyethane and tetrahydrofuran, and containing various stabilizing additives.
Important features of these cells include their ability to be repeatedly discharged and charged. Theoretically, cycling by discharging and charging should be possible indefinitely, but in practice indefinite cycling is not realized. Dendritic growth on the anode during charging and degradation of the cathode material are sometimes limiting factors in the amount of cycling to which a cell can be subjected. However, the electrolyte, particularly nonaqueous electrolytes, can at times be the limiting factor. The effects of a particular electrolyte composition on the electrochemical performance of a cell may be significant due to its relative stability or it may be due to other factors. One particular electrolyte composition might be highly effective with a given anode-cathode couple but be ineffective for another couple, either because it is not inert to the second couple or because it reacts with itself under the conditions present during cycling. Furthermore, even when a particular electrolyte composition is effective in a given cell, it may nonetheless be undesirable for other reasons. For example, the sometimes preferred LiClO.sub.4 based electrolyte creates a potential explosion hazard. And, for example, various organometallic alkali metal salt compounds such as are described in U.S. Pat. Nos. 3,734,963 and 3,764,385 have the disadvantage of requiring complexing with various nitrogen, phosphorus or sulfur-containing organic compounds containing at least two functionalities. Recent studies have been made directed to LiB(C.sub.6 H.sub.5).sub.4 electrolyte systems by Bhattacharyya et al, J. Phys. Chem., Vol. 69, p. 608 et seq. (1965) but these systems have been found to have low solubility and high resistivity. Additionally, mention has been made by A. Brenner that certain alkali metal organometallic salts have electrochemical properties, e.g., NaB(C.sub.2 H.sub.5).sub.4 and NaAl(C.sub.2 H.sub.5).sub.4, in Advances in Electrochemistry and Electrochemical Engineering, Vol. 5, at page 214. Also, Chambers et al, JACS, Vol. 82 (Oct. 20, 1960), pages 5298-5301, and Burger et al, Inorganic Chem., Vol. 16, No. 9 (1977), pages 2305-2314 describe various alkali metal salts of the metaloorganic structure, including the fluorinated compound KB(CF.sub.3)F.sub.3. Vandeberg et al in Anal. Chimica Acta, Vol. 44, pp 175 et seq. (1969), describe various sodium salts such as NaB(C.sub.6 H.sub.4 --p--F).sub.4, NaB(C.sub.6 H.sub.4 --m--F).sub.4 and NaB (C.sub.6 H.sub.4 --p--CF.sub.3).sub.4. However, none of these references teach any electrochemical utility for these compounds or suggest that the compounds used in the present invention even exist. Massey et al, J. Organomet. Chem., Vol. 2 (Feb. 14, 1964) pp. 245-250, describe the compounds LiB(C.sub.6 F.sub.5).sub.4 and KB(C.sub.6 F.sub.5).sub.4 but do not teach their use in electrolyte systems, much less in the specific cells of the present invention.
Seyferth et al, J. Organometallic Chemistry, Vol. 141, pp. 71-83 (1977) describe the preparation of LiB(C.sub.6 H.sub.5).sub.3 (CH.sub.2 CH.dbd.CCl.sub.2). However, this compound differs from the present invention compounds in that it is unsaturated. The substituents on the central metal atom of the compounds described herein do not include halogenated alkenyl groups. Ahmed et al, Inorganic Chemistry, Vol. 8, pp. 1411-1413 (1969) describe [(C.sub.2 H.sub.5).sub.4 N].sup.+ [B(C.sub.6 H.sub.5)Cl.sub.3 ].sup.- in acetonitrile and report its conductivity, but do not teach an alkali metal analogue. Muetterties et al, Inorganic Chemistry, Vol. 4, pp. 119-121 (1965) use NMR to infer the existence of H.sup.+ [P(CH.sub.3)F.sub.5 ].sup.- and H.sup.+ [P(C.sub.6 H.sub.5)F.sub.5 ].sup.- in dimethylsulfoxide solution (CH.sub.3 SOCH.sub.3), but do not teach the existence of alkali metal analogues. Chan et al, Canadian Journal of Chemistry, Vol. 46, pp. 1237-1248 (1968) describe the preparation of Cs.sup.+ [P(CF.sub.3)F.sub.5 ].sup.-, and Cs.sup.+ [P(CF.sub.3).sub.2 F.sub.4 ].sup.-, and Cs.sup.+ [P(CF.sub.3).sub.3 F.sub.3 ].sup.-, and Ag.sup.+ [P(CF.sub.3).sub.2 F.sub.4 ].sup.- and their properties in water and acetonitrile. However, no alkali metal analogues are described or suggested. U.S. Pat. No. 4,060,074 describes various alkali metal-metal-organic salts and their use in electrochemical cells, but haloorganic substituted salts are not taught.
In summary, it is believed that to date the described cells of the present invention containing the improved electrolyte compositions including haloorganometallic alkali metal complexes, have not been heretofore disclosed or rendered obvious.